PoC (Push-to-talk over Cellular, PoC for short) is a push-to-talk service based on a public cellular mobile communication network. A user needs only to press one key like using a walkie-talkie to quickly and directly perform one-to-one or one-to-multiple instant communication with one or multiple users in any place without a need to dial a number. A PoC system includes a PoC client (PoC Client), a PoC server (PoC Server), and an Online Charging System (Online Charging System, OCS for short). The PoC client residing in a mobile terminal is used to get access to a PoC service, and support session initiation, participation, and termination. The PoC server is divided into a PoC Controlling Function (PoC Controlling Function, PoC CF for short) and a PoC Participating Function (PoC Participating Function, PoC PF for short). The PoC CF exercises centralized control on a session, including media distribution, controlling the floor, executing a session policy on members that participate in a session, and processing information about participating members. The PoC PF provides session control, including exercising policy control on incoming sessions, relaying floor control signaling between the PoC CF and the PoC client, and so on. Depending on a requirement, the PoC PF may also provide relaying of a media stream and store a user's service settings such as an answer mode and incoming call barring. In PoC 2.0 version standard and earlier versions, a dedicated channel (Normal PoC Channel) is used. That is, for each PoC client, an uplink radio channel and a downlink radio channel exist logically. The uplink radio channel is configured to send a talk burst (Talk Burst) or a media burst (Media Burst) to the PoC server, and the downlink radio channel is configured to receive the talk burst or the media burst. Each PoC client physically occupies an independent radio channel to send and receive the talk burst (Talk Burst) or the media burst (Media Burst). With an increase of users, loads on the PoC server and a wireless network increase rapidly, which affects conventional voice services and data services. To further optimize usage of wireless network resources, a multicast channel (Multicast PoC Channel) is introduced into PoC 2.1 version standard. That is, in a PoC session, when the number of PoC clients in one cell reaches a specified number, the PoC server sets up a multicast channel, where the multicast channel is exclusively used for receiving a downlink talk burst or media burst. With the multicast channel in use, for each PoC client, an uplink radio channel exists; for PoC clients in the same cell in a session, a downlink radio channel is shared.
In PoC 2.0 version standard and earlier versions, the PoC client gets access to the PoC service only through a dedicated channel. Therefore, the OCS charges only a PoC client that uses the dedicated channel. When the channel through which the PoC client gets access to the PoC service is switched during the course of using the service by the PoC client, the OCS is unaware of the switching of the channel through which the PoC client gets access to the PoC service and unable to perform precise and proper charging for the PoC client that switches the channel through which the PoC client gets access to the PoC service.